Device with a plasma torch

ABSTRACT

A device with a plasma torch with a rod-shaped non-consumable electrode ( 3, 4 ) which is held in a receiver ( 1 ), is connected with an electric connection and penetrates a nozzle ( 9 ) which is in connection with a gas connection. In order to also enable the rapid and secure welding of difficult alloys it is provided that a further rod-shaped non-consumable electrode ( 3, 4 ) is disposed in the receiver ( 1 ) which is made from an electrically non-conducting material, which further electrode also penetrates a nozzle ( 9 ′) which is in connection with a gas connection, with the two electrodes ( 3, 4 ) enclosing an acute angle and each being in connection with a separate voltage source ( 31, 32 ) supplying direct voltage pulses, the level of which exceeds at least the arc voltage of an arc between one of the electrodes ( 3, 4 ) and a counterelectrode which is associated with the same and connected with the same voltage source ( 31, 32 ).

[0001] The invention relates to a device according to the preamble ofclaim 1.

[0002] A device of the kind mentioned above with merely one rod-shapedelectrode is used for the welding of light metal and light metal alloys.In order to achieve a high welding speed at deep fusion penetration andnarrow seams, the rod-shaped electrode is switched as a cathode andhelium is used as a plasma gas. A very hot plasma is obtained whichevaporates thin oxide layers. This is not the case in all light metalalloys, however.

[0003] In order to also enable the welding of such alloys, welding isperformed with direct current instead of alternating current, or theelectrode is applied to the plus pole of the voltage source. Although acontinual removal of the oxide layers is ensured and a welded joint isensured which is free from cavities because the oxide skin iscontinuously torn open, this advantage is offset by the disadvantage ofa welding speed which is reduced by approximately two-thirds as comparedwith a d.c. helium welding and a considerable increase of the width ofthe weld seams with an increased heat influence zone.

[0004] It is the object of the present invention to avoid suchdisadvantages and to provide a method of the kind mentioned above whichallows a high welding speed also in difficult alloys and which alsoensures that any arising oxide layers are removed.

[0005] This is achieved by a method of the kind mentioned above by thecharacterizing features of claim 1.

[0006] As a result of the proposed measures it is possible to connectthe two rod-shaped electrodes with different poles of the voltagesources. As a result, plasma pulses which are produced with an electrodeconnected to the plus pole of a voltage source can be used to tear openthe oxide layers and with the subsequent plasma pulses which areconnected with the minus pole of a voltage source and are thereforeproduced by an electrode switched as a cathode it is possible to weldthe basic material in a clean manner and with a high penetration depth,with very narrow and smooth weld seams being obtained. By locking theswitching devices which each only allow voltage pulses of approx. 1 to 5milliseconds, it is ensured that only one electrode can be charged.

[0007] The workpiece to be worked can appropriately be switched as thecounterelectrode. It is also possible to make the nozzle or therespective nozzle body from an electrically conductive material and toswitch the same as a counterelectrode.

[0008] In the case of alloys that can be welded more easily, bothelectrodes can also be switched as cathodes. This leads to the advantagethat the required welding energy can be divided among both electrodesand they can therefore be provided with a thinner arrangement. Thisallows the production of very narrow receiving means of 9 mm width forexample. With such devices it is therefore also possible to weld incorner zones of workpieces which are difficult to access, whichsubstantially facilitates the constructional design of such pieces.

[0009] As a result of the two separate voltage sources they can also becontrolled with respect to the pulse length and pulse power, thusenabling a highly substantial adaptation to the respective requirements.

[0010] The ignition of each plasma arc can be made by means of ahigh-frequency pulse when the level of the voltage of the individualvoltage pulses does not exceed the respective breakdown voltage of thepath between the electrode and the respective counterelectrode. Theignition can also be initiated per se by respectively high voltagepulses which exceed the respective breakdown voltage.

[0011] Very favorable conditions for the welding of very difficultalloys are obtained by the features of claim 3. It has also proven to beadvantageous to also provide the features of claim 2, with the electrodeswitched as a cathode preferably standing perpendicularly to theworkpiece.

[0012] The features of claim 4 allow keeping the wear and tear of theelectrode, which is connected with the plus pole and is subject tohigher stresses, at a very low level.

[0013] The features of claim 5 provide a simple arrangement of thereceiving means. It must be ensured however that the higher stressedelectrode is cooled sufficiently well.

[0014] The measures according to claim 6 allow achieving an ionizationof the plasma gas flowing from the nozzle in the zone between theelectrode and the nozzle as a result of a high-frequency arc-over and,as a result, the ignition of an arc between the electrode and theworkpiece as a result of the applied direct voltage. This leads to asubstantial protection of the plasma torch, because the same is notencumbered by the otherwise common pilot arc.

[0015] As a result of the ionization by the high-frequency arc-over,which imposes only very low thermal stresses on the nozzle, it is alsopossible when using helium as a plasma gas to easily ignite over largerdistances between the electrode and workpiece of 10 mm for example.

[0016] The use of a nozzle which is made from an electricallywell-conducting material and its connection via a high-resistanceelectric resistor with the pole of the voltage source which is connectedwith the workpiece is also of advantage in devices in accordance withthe invention in which the plus pole of the voltage source is connectedwith the electrode penetrating the nozzle.

[0017] The features of claim 8 allow using the device in a veryuniversal manner.

[0018] The features of claim 9 and 11 produce a constriction of theplasma and avoid a divergence of the same due to the friction of theplasma in the air which emerges with a high speed, so that a very highconcentration of the energy is achieved.

[0019] The invention is now explained in closer detail by reference tothe enclosed drawing, wherein:

[0020]FIG. 1 shows a sectional view through a first embodiment of thedevice in accordance with the invention;

[0021]FIG. 2 shows a cross-sectional view through the device accordingto FIG. 1;

[0022]FIG. 3 shows a sectional view through a second embodiment of thedevice in accordance with the invention;

[0023]FIG. 4 shows a top view of the device according to FIG. 3;

[0024]FIGS. 5 and 6 show a device according to FIGS. 3 and 4 with apower supply unit, shown in a partial sectional view, in a projectionand top view;

[0025]FIG. 7 shows a detail of the nozzle area;

[0026]FIG. 8 schematically shows the electric power supply of thedevice;

[0027]FIG. 9 shows a diagram of the progress over time of the voltagecharging of the electrodes of a device in accordance with the invention;

[0028]FIG. 10 shows a variant of the embodiment according to FIGS. 1 and2 in a sectional view.

[0029] A receiver 1 is provided in the embodiment according to FIGS. 1and 2, which receiver is made from an electrically insulating material.Two holding devices 2 are inserted in said receiver 1, at the end ofwhich there are two electrodes 3, 4 made of a thermally stable materialsuch as tungsten for example.

[0030] The holding devices 2 are made of an electrically well-conductingmaterial and are provided with a central bore 5 which are connected inthe upper and lower range via radial bores 6 with chambers 7, 8, ofwhich the chambers 7 are each connected with a gas conduit 109, 109′through which plasma gas can be supplied separately, and the chambers 9are each connected with an ejection nozzle 9, 9′.

[0031] Said nozzles 9, 9′ are provided with conical inner walls, withthe inner wall of nozzle 9 extending substantially parallel to theconical end zone of electrode 3, whereby the free end of the electrode 3can be flattened. The electrode 5 is provided with a substantially bluntarrangement in contrast to electrode 3.

[0032] Furthermore, a cooling conduit 10 is further provided in thereceiver 1, which conduit leads from an inlet 11 to an annular chamber12 which is penetrated by holding device 2 of the electrode 4, and fromthe same, divided into two branch conduits (FIG. 2), to a furtherannular chamber 13 which is penetrated by the holding device 2 of theelectrode 3 and from the same to an outlet 14.

[0033] The electric connection of the two electrodes 3, 4 or theirholding devices 2 can be provided through screw caps 15, or if the gasconduits 109, 109′ are provided with electrically conducting walls, viathe same. In the latter case the connection can be made throughconnecting nipples through which gas is supplied.

[0034] In the embodiment according to FIGS. 1 and 2, a tubular guidemeans 16 is provided between the nozzles 9, 9′, which guide means isprovided for guiding a wire used as an additional material. The guidemeans 16 is offset.

[0035] As can be seen from FIG. 2, the receiver 1 can be provided with avery narrow arrangement.

[0036] In the embodiment according to FIGS. 1 and 2, the electrode 3extends in the position for use of the receiver 1 in a substantiallyvertical manner and the electrode 4 encloses with the same an acuteangle which can usually be 20° to 70°.

[0037] Two similar electrodes 3 are provided in the embodiment accordingto FIGS. 3 and 4, which electrodes both enclose an angle with theperpendicular.

[0038] As is shown in FIG. 5, the receiver 1 is provided withflange-like projections 16 which are penetrated by screws 17 with whichthe receiver 1 can be fastened to a connecting head 18, with the screws17 engaging in threaded bores 19 of the connecting head 18.

[0039] Spring-biased connecting nipples 30 are held axially displaceablein said connecting head 18, to which a water supply line 21 and a waterdischarge line 22 for supplying and discharging cooling water areconnected, with said spring-biased connecting nipples 20 engaging, whenthe receiver 1 is closed, in the inlet and outlet 11, 14 of the same.Fixed connecting nipples 23 are further provided in said connecting head18 to which gas lines 24 are connected which convey helium for example.The fixed connecting nipples 23 engage in the inlets 25 of the gasconduits 109, 109′ when the receiver 1 is closed. O-rings are used forsealing in the inlets 25, as in the inlet and outlet 11, 14.

[0040] Furthermore, a pin 26 which is arranged off-center is held in theconnecting head 18, which pin engages in a respective bore 27 of areceiver 1. This ensures that a connection of a receiver 1 to theconnecting head is only possible in a certain position in which thecorrect flow of the gas and cooling conduits is given.

[0041] Receivers 1 which are equipped with different electrodes 3, 4 canbe connected to the connecting head 18. Such an exchange can beperformed very simply.

[0042]FIG. 7 shows a detail of the nozzle body 9 for an electrode 3which is provided with a conical or substantially tapered end. The innerwall 27 of the nozzle body 9 extends substantially parallel to theconical end of electrode 3. This measure ensures that the plasma gasemerges directed in an inclined manner against the axis of nozzle 9,therefore counteracting the tendency of the emerging plasma to divergewith increasing distance from the orifice of nozzle 9 owing to frictionin the ambient air. As a result, only a small arc spot is obtained in adesired manner on the workpiece to be processed.

[0043] Cold gas conduits 29 are provided in the nozzle body 9 andenclose its conical nozzle bore 28. Said conduits are evenly distributedconcentrically about the nozzle bore 28. The axes of said cold gasconduits 29, most of which are provided in odd numbers such as 3, 5 or7, form a generatrix of a conical surface whose axis lies concentricallyto the axis of the nozzle bore 28. Said cold gas conduits are opentowards the chamber 8 and open at the face side of the nozzle body 9.

[0044] The plasma gas flowing through these cold gas conduits produces acooling of the nozzle body 9 on the one hand and a further constrictionof the plasma emerging from the nozzle 9 on the other hand and thus areduction of the arc spot and thus an increase in the energyconcentration in the same. The supply of the chamber 8 with plasma gasis performed through a gas conduit 109, 109′, the upper radial bores 6of the holder 2, its central bore 5 and the lower radial bores 6.

[0045]FIG. 8 schematically shows the connection of the device inaccordance with the invention. The electrodes 3, 4 are each connectedwith a pole of a voltage source 31, 32 each whose respective second poleis connected via a switching device 33, 34 each to a workpiece 30.

[0046] The two switching devices 33, 34 are mutually locked, so thatonly one switching device 33 or 34 each can be switched through. Onlyshort switch-through times are provided for the two switching devices33, 34, so that the electrodes 3, 4 can only be charged in pulses.

[0047] For numerous applications the electrode 3, which is disposed atthe back as seen in the welding direction, is switched as a cathode andconnected to the minus pole of the voltage source 32.

[0048] Typical values are a current application of approx. 170 A for atime of approx. 15 ms each and a break of approx. 3 ms. During this timethe switching device 33 switches through and the electrode 4 connectingthe plus pole of the current source 31 is charged with approx. 250 A forapprox. 3 ms.

[0049] With such a mode of operation it is also possible to favorablyand rapidly weld even alloys which are difficult to weld, because bycharging the electrode 4 the plasma pulses thus produced will securelytear open any oxide skins and the basic material can be welded veryfavorably with the subsequent plasma pulses which are produced bycharging the electrode 3.

[0050] For certain applications it is also possible to insert twoelectrodes 3 in the receiver 1 and to connect both with the minus poleof one direct voltage source 32 each and to charge the samesubstantially alternatingly. Overlap periods can also be provided,however. Since in this way the stress on each electrode 3 isrespectively low, electrodes 3 with a small diameter can be used, thusenabling the construction of the receiver with a narrow design.

[0051] The embodiment according to FIG. 10 differs from the one of FIGS.1 and 2 in such a way that helical ribs 35 are arranged in the chambers8 which are in connection with the gas connections through the gasconduits 109, 109′, with helically extending conduits remaining betweensaid helical ribs through which the plasma gas flows to the nozzles 9,9′.

[0052] The same is subjected to a twist which leads to a stabilizationof the plasma emerging at a high speed from the nozzles 9, 9′, thussubstantially preventing any divergence of the plasma due to friction inthe substantially static air and thus leading to a very small arc spotwith high energy density on the workpiece 30 to be processed.

1. A device with a plasma torch with a rod-shaped non-consumableelectrode (3) which is held in a receiver (1), is connected with anelectric connection and penetrates a nozzle (9) which is in connectionwith a gas connection, characterized in that a further rod-shapednon-consumable electrode (3, 4) is disposed in the receiver (1) which ismade from an electrically non-conducting material, which furtherelectrode also penetrates a nozzle (9′) which is in connection with agas connection, with both electrodes (3, 4) enclosing an acute angle andeach being in connection with a separate voltage source (31, 32)supplying direct voltage pulses, the level of which exceeds at least thearc voltage of an arc between one of the electrodes (3, 4) and acounterelectrode associated with the same and connected with the samevoltage source (31, 32), with the voltage supply being performed atdifferent polarization of the two electrodes (3, 4) via mutually lockedswitching devices (S1, S2) which prevent any simultaneous voltagecharging of the two rod-shaped electrodes (3, 4).
 2. A device as claimedin claim 1, characterized in that the two rod-shaped electrodes (3, 4)enclose an angle of 20° to 70°, preferably 30°.
 3. A device as claimedin claim 1 or 2, characterized in that one pole each of each voltagesource (31, 32) can be connected to a workpiece (30) to be processed andthe two rod-shaped electrodes (3, 4) are connected to different poles ofthe two voltage sources (31, 32), with the electrode (4) which isconnected to a plus pole being disposed at the front as seen in thedirection of welding.
 4. A device as claimed in one of the claims 1 to3, characterized in that the electrode (4) connected to a plus pole isprovided with a substantially blunt arrangement in the zone of its freeend projecting from the nozzle (9′), whereas the electrode (3) connectedwith a minus pole being provided with a substantially conicalarrangement in the zone of its free end.
 5. A device as claimed in oneof the claims 1 to 4, characterized in that a cooling conduit (10) isprovided in the receiver (1), which conduit mutually connects chambers(8) with one another which are arranged in the receiver (1) and arepenetrated by the rod-shaped electrodes (3, 4) or their holders (2) andwith an inlet (11) and an outlet (14), with the chamber (8) which ispenetrated by the electrode (4) which is connected to the plus polebeing connected to the inlet (11).
 6. A device as claimed in one of theclaims 1 to 5, characterized in that at least the nozzle (9) which ispenetrated by an electrode (3) which is connected to the minus pole ofthe respective voltage source (32) is made of an electricallywell-conducting material and is connected via a high-resistance electricresistor (1R2) in the range of 10³ to 10⁶ ohms, preferably 10⁵ ohms,with the pole of the voltage source (32) which is connected with theworkpiece (30).
 7. A device as claimed in one of the claims 1 to 5,characterized in that at least the nozzle (9) which is penetrated by anelectrode (3) which is connected to the plus pole of the respectivevoltage source (32) is made from an electrically well-conductingmaterial and is connected via a high-resistance electric resistor (1R2)in the range of 10³ to 10⁶ ohms, preferably 10⁵ ohms, with the pole ofthe voltage source (32) which is connected with the workpiece (30).
 8. Adevice as claimed in one of the claims 1 to 7, characterized in that aguide means (16) for a wire used as additional material is provided inthe receiver (1) between the nozzles (9, 9′) penetrated by therod-shaped electrodes (3, 4).
 9. A device as claimed in one of theclaims 1 to 8, characterized in that the electrode (3) connected with aminus pole of a voltage source (32) penetrates a conical nozzle (9) withits conical free end zone, with the conical surfaces of the nozzle (9)and the electrode (3) extending substantially parallel with respect toone another and being provided substantially with a cone angle ofapprox. 20°.
 10. A device as claimed in one of the claims 1 to 9,characterized in that the electrode (4) connected with a plus pole of avoltage source (31) penetrates a substantially cylindrical nozzle (9′)and a substantially constant annular gap remains between its inner walland the electrode (4).
 11. A device as claimed in one of the claims 1 to10, characterized in that the bodies of the nozzles (9) are providedwith cold gas conduits (29) which are in connection with the respectivegas connection and are arranged concentrically to the axis of the nozzlebore (2) and evenly distributed about the same and are open at the freeface side of the nozzle (9), with the axes of said cold gas conduits(29) forming generatrices of a conical surface whose tip is disposedbefore the free end of the electrode (3).
 12. A device as claimed in oneof the claims 1 to 10, characterized in that helically extending ribs(30) which define helical conduits are arranged in a chamber (8) whichis in connection with a gas connection and is penetrated by an electrode(3, 4) or its holder (2).